A. Field of the Invention
This invention relates to dashpots, and, in particular, to a dashpot mechanism for a weighing or load cell.
B. Prior Art
Weight or load cells are known which respond to pressure applied thereto by generating an electrical signal which is a function of the displacement of a movable element of the cell. One type of weigh cell marketed by Automatic Timing and Controls Co. of King of Prussia, Pa. has an element fixedly attached to a weighing platform that communicates axially with a vertically displaceable O-shaped bracket. The bracket is connected at its top and bottom by two horizonal metallic flexure members or plates to respective fixed points within the chassis. Movement of the bracket is accomplished against the flexing force of the plates and of a precision spring that restores the platform and the bracket to their original "tare" position. Attached to the bracket is one element of a signal-generating apparatus such as a linear voltage differential transformer. This movable element, which may be the armature thereof, passes coaxially through the center of the coils of the transformer. Small changes in the displacement of the bracket, and hence the armature, under load cause changes in the coupling of a signal applied to the primary winding of the transformer relative to its secondary so that the output signal has a characteristic which is a function of the bracket displacement.
In the weigh cell art, it has been desirable to incorporate damping mechanisms to prevent undesired oscillation of the movable bracket-armature assembly. One such damping mechanism employed in the above-mentioned construction involved the coaxial mounting of two truncated circular and coaxial disks as the resistance element or piston. It reciprocated within the dashpot cylinder containing the damping material, i.e., the resistance liquid. When these two disks were oriented at right angles to one another they produced effectively a circular piston head for maximum damping. Since there was a slight clearance between the composite circular configuration and the internal wall of the cylinder, damping liquid could move in the clearance opposite the direction of the piston stroke. One of the shafts to which one disk was attached had a slotted end which could be engaged by a screwdriver from outside the chassis to permit rotation of that shaft to the desired angular orientation. When the disks were disposed at congruous positions, minimum damping was accomplished. Intermediate rotary positions of the disks brought about intermediate degrees of damping.
While the above arrangement was adequate for many purposes, the size of the truncated disks did not allow much resolution of degrees of damping. Furthermore, continued movement of the slotted member in the same direction produced first, increased resistance, to a maximum and then, decreased resistance to a maximum and so on. Consequently, the position of the slotted shaft was little help in determining which direction to turn at any given moment. This aspect of the prior art made for ambiguity and was time-consuming when initial installation or maintenance calls were made.
In addition, there were no positive mechanical means for stopping at the maximum and minimum resistance points.
The geometry of this two-part piston system as embodied in those weigh cells limited the damping capacity of their dashpot mechanism. This, in turn, made the possibility of oscillation higher which adversely affects the accuracy as explained in an article by V. Whittaker in the March, 1975 issue of "Measurements and Data".
It is, therefore, among the objects of the present invention to provide a damping subassembly especially for use with weigh or load cells or the like which has
1. Greater resolution than prior damping subassemblies. PA1 2. A more ascertainable, stepped resolution characteristic. PA1 3. A less ambiguous variable damping positional characteristic. PA1 4. Positive mechanical means for limiting maximum and minimum dashpot piston excursions. PA1 5. A larger damping capacity than is available in many commercially available ones. PA1 6. Greater ease of adjustment of the damping from outside of the cell itself. PA1 7. An easily installed and easily removable dashpot chamber.